The overall objective of this project is to gain a sufficient depth of knowledge of the variables essential to the maintenance of arboviral transmission cycles to allow effective control programs. This objective is approached through a series of interrelated projects to establish: the epidemiologic characteristics of cases, bionomics of vectors and hosts, evaluation of intrinsic and extrinsic factors that control the competence of mosquitoes as vectors and the impact of physical and biological factors on success of virus cycles. Success in the overall objective is measured in terms of our ability to lessen the risk of clinical disease (encephalitis) in humans and horses that live in endemic regions.